Writing a Base$64$ Decoder program using MIPS with the format below:

#

# base$64$decode.s $($incomplete$)$

#

# by Deborah Pickett $2003-02-27$

#

# This MIPS program reads lines of Base $64-$encoded text from standard

# input, and outputs the decoded bytes to standard output.

#

# INSERT YOUR CODE AT THE POINT INDICATED BELOW.

#

# Data segment

#

$.$data

# Space to read a line into.

inbuffer: $.$space $80$

# The Base $64$ alphabet, in order.

sequence: $.$asciiz "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz$0123456789+/"$

#

# Text segment

#

$.$text

# Program entry.

main:

# The first byte we're expecting is byte $0$ of a group of $4.$

la $t$9,$ byte$0$

# Read a string from standard input.

loop: li $v$0,8$

la $a$0,$ inbuffer

li $a$1,80$

syscall

# Is this an empty line? Since SPIM can't detect when end of

# file has been reached, we need to use another way to indicate

# the end of the Base $64$ data. We'll use a completely

# blank line for this.

lb $t$0,$ inbuffer

# First character newline means there was no text on this line,

# so end the program.

beq $t$0,10,$ alldone

# Walk along the string. Start at the beginning.

la $t$8,$ inbuffer

# Go back to where we left off last time $($byte $0,1,2$ or $3).$

jr $t$9$

# Get four characters at a time.

byte$0$: lbu $s$0,0($$t$8)$

add $t$8,$ $t$8,1$

beq $s$0,10,$ linedone

# Now up to byte $1.$

la $t$9,$ byte$1$

byte$1$: lbu $s$1,0($$t$8)$

add $t$8,$ $t$8,1$

beq $s$1,10,$ linedone

# Now up to byte $2.$

la $t$9,$ byte$2$

byte$2$: lbu $s$2,0($$t$8)$

add $t$8,$ $t$8,1$

beq $s$2,10,$ linedone

# Now up to byte $3.$

la $t$9,$ byte$3$

byte$3$: lbu $s$3,0($$t$8)$

add $t$8,$ $t$8,1$

beq $s$3,10,$ linedone

# Now all bytes in this block are read.

# Four Base$64$ characters are now in $s$0,$ $s$1,$ $s$2,$ $s$3.$

bytesdone:

#

# DO NOT DELETE THIS LINE.

######

#

# PUT YOUR ANSWER HERE.

# Your answer should not modify $t$8$ or $t$9,$ as they are used by

# the above code.

#

######

# DO NOT DELETE THIS LINE.

#

endgroup:

# Go back to do next bunch of four bytes. We're now expecting

# byte $0$ of $4.$

la $t$9,$ byte$0$

j byte$0$

linedone:

# Line is finished; go get another one.

j loop

alldone:

# Exit.

li $v$0,10$

syscall